A group III nitride-based compound semiconductor light-emitting devices whose light emitting layer or active layer have a multiple quantum well structure have been widely used as a light-emitting diode or a laser diode which emits a light with any spectrum in the range from purple to green.
A multiple quantum well structure means, in principle, a periodical structure of the two layers comprising a well layer which is an emission body with the narrowest band gap and a barrier layer whose band gap is wider than the well layer. However, a multiple quantum well layer structure encompasses a periodical structure of three layers having, for example, a well layer, a protective layer or the other layers and a barrier layer.
For example, a Group III nitride-based compound semiconductor light-emitting device is known for the periodical structure of an unit which comprises a well layer comprising InGaN, a protective layer comprising GaN which is formed at the same growth temperature as a well layer is formed at and prevents In from evaporating from just below the well layer, and a barrier layer comprising AlGaN as MQW.
In general, the role of a protective layer comprising GaN is as below.
As has been known, the epitaxial growth temperature of the layer comprising In is less than or equal to 850° C., preferably less than 800° C. The reason to keep the growth temperature low is that In—N binding easily disassociates under the growth of high temperature, that is, In atom is disappeared, and the layer having the desired composition can not be formed. The composition of the well layer on the light-emitting device is an important factor for determining the wavelength of an emitted light. Consequently, in order to stabilize In composition of the well layer, the growth temperature of the well layer is kept less than 800° C.
On the other hand, the barrier layer of the multiple quantum well structure is necessary to confine carriers in the well layer, therefor an energy barrier of the barrier layer should be sufficiently high. The layer comprising Al is employed for this barrier layer. However, when the semiconductor comprising Al is grown under less than 800° C., the semiconductor of high crystallinity can not be obtained. In order to obtain the barrier layer comprising Al of fine crystallinity, the barrier layer should be formed at the temperature of equal to or more than 850° C.
However, when the well layer comprising In is epitaxially grown at the temperature of less than 800° C., then the barrier layer comprising Al is epitaxially grown at the temperature of equal to or more than 850° C., the barrier layer can not be grown in a succession of growth of the well layer. That is, the barrier layer should be grown after elevating a temperature from the growth temperature for the well layer to an optimal growth temperature for the barrier layer. During the heating-up period, however, an atomic binding of the surface of the well layer comprising In which was formed in first was dissolved and In atom was dissociated from the well layer. Accordingly, the composition of the surface of the well layer comprising In which was formed in desired composition was once changed before the barrier layer comprising Al was started to be formed, and moreover, roughness were formed on the surface of the layers. This exerted an important influence on the characteristics of the light-emitting device.
Then, after the well layer comprising In was epitaxially grown at the temperature of less than 800° C., for example, a protective layer comprising GaN was formed on the well layer at the same temperature. Thereby In dissociation from the well layer comprising In was prevented.    [Patent documents 1] unexamined laid open JP, 2001-237456    [Patent documents 2] unexamined laid open JP, 2000-261106